Most solid-state gas sensors available to date are based on the electrical response of the solid to its chemical environment. That is, the electrical properties of the solids are affected by the presence of a gas-phase species, and this change is employed to detect the species. These solid-state sensors can be divided into three classes: semiconductor sensors where the species to be detected is adsorbed or absorbed and changes the electronic conductivity of the semiconductor; solid electrolyte sensors for use in gas, where the species to be detected affects the Nernst potential or changes the ionic current through the solid; and field-effect-transistor gas sensors (ChemFET), in which the species to be detected affects the potential at the gate of a field-effect transistor.
The semiconductor sensors based on pressed powders and thin films are commercially available. The sensors are designed based on a reaction between the semiconductor and the gases in the atmosphere, which produces a change in semiconductor conductance. One possible reaction is that in which the semiconductor is converted to another compound, or at least to another stoichiometry. For example, one can have a semiconducting oxide which is oxidized by oxygen from the atmosphere, but lattice oxygen is extracted when some organic vapors are introduced into the atmosphere. Thus the presence of the organic vapor lowers the cation/oxygen ratio in the oxide--that is, it changes the stoichiometry of the solid. Such a stoichiometry change (indeed, any change in the composition of the solid) can have a significant effect on the conductivity of the material.
More commonly, with semiconductor gas sensors the "reaction" leading to conductivity changes is considered to be the adsorption of gases. The effects of the gaseous ambient are interpreted to be due not to changes in bulk composition but to adsorption gases on the surface of the semiconducting solid. In the adsorption mechanism, the usual model is as follows: Oxygen from the atmosphere adsorbs and extracts electrons from the semiconductor. If the solid conducts by electrons, the conductivity will decrease as the electrons are extracted. When an organic vapor is present in the atmosphere, it reacts with the negatively charged oxygen, becoming oxidized, perhaps to H.sub.2 O and CO.sub.2, and the electrons are returned to the solid, restoring the conductivity. Consequently, the conductivity is much higher with the organic vapor present in air than it is for pure air. Of course, sometimes it is difficult to tell which process--stoichiometry changes or adsorption--affects the conductivity change. In most cases, however, the change is sufficiently rapid compared to the expected diffusion rate of the oxygen vacancies (or other species which diffuse) in the solid, so that one can be reasonably certain that the conductivity changes can not be due to changes in bulk composition.
The third possible reaction between the semiconductor and the gas is ion exchange near the surface, a process intermediate between the other two. For example, a surface ion might replace an oxide ion at the surface of the metal oxide semiconductor in the presence of H.sub.2 S vapor in the atmosphere. Since sulfides often are much more conductive than oxides, such an exchange may lead to a high surface conductivity.
In solid electrolytes the conductivity stem from mobile ions rather than electrons. Typically the conductivity is dominated by one type of ion only. Solid electrolytes already play an important role in commercial gas and ion sensors. In these applications solid electrolytes are used as nonporous membranes separating two compartments containing chemical species at different concentrations on either side thereof. By measuring the potential across such a membrane, one can determine the concentration of the chemical species on one side if the concentration on the other side (i.e., the reference side) is known. In general the solid electrolytes allow the quantitative determination of the concentration of those species that are ionically transferred in the electrolyte.
The ChemFET is a class of sensors developed recently as variations on field-effect transistors (FETs). In a FET one has a thin channel of conductance at the surface of the silicon, which is controlled by voltage applied to a metal film (a gate) separated from the channel of conductance by a thin insulator layer (e.g., silicon dioxide) It has been found that if the metal film was removed from the FET and either adsorbed gases or ions from the ambient atmosphere appeared at the surface of the gate dielectric, the effect was similar to applying a voltage at the gate. Selectivity can be induced in these sensors by appropriate incorporation of, for example, certain pH-sensitive insulators and ion-sensitive membranes in ion-sensitive field-effect transistors (ISFETs).
Based on the above principles, various solid-state gas sensors have been invented, and their descriptions are available in the patent and other technical literature. For example, U.S. Pat. No. 4,903,099 describes a ChemFET type gas sensor; U.S. Pat. Nos. 4,896,143, 4,706,493, 4,169,369 and 5,143,696 disclose semiconductor-type gas sensors; U.S. Pat. Nos. 4,025,412, 4,227,984, 4,394,239, 4,522,690, 5,302,274, and 5,173,166 disclose electrolytes-type gas sensors; and U.S. Pat. Nos. 5,191,784, 3,450,620 and 5,184,500 disclose some other type of gas sensing devices. Most of these solid-state gas detectors, however, have one or more disadvantages associated therewith, including: (a) sophisticated structure and relatively large size, (b) limited range of detectable gases, (c) limited range of operatable temperature, (d) elevated operation temperature, (e) low sensitivity, (f) slow response, with hysteresis, (g) expensive manufacturing cost, (h) low reliability, (i) installation and operation inconvenience, and (j) low integratability with signal processing integrated circuit (IC).
Accordingly, there exists a need in many industries for enhanced solid-state gas sensors, having the following characteristics: (a) simple structure and small dimension, (b) wide operating range of detectable gas, (c) wide operation range of temperature, (d) atmospheric temperature operability, (e) high sensitivity, (f) rapid response without hysteresis, (g) low manufacturing cost, (h) high reliability, (i) installation and operation convenience, and (j) integratability with signal processing IC.